This invention relates generally to intermediate frequency circuits and more particularly to digital intermediate frequency circuits where the intermediate frequency is zero Hertz.
Traditionally, intermediate frequency (IF) sections have been employed in transmitters and receivers to perform the major portion of a radio's selectivity since it may be technically difficult, or cost prohibitive, to develop sufficiently selective filters at the transmitted or received frequency. Transceivers may have more than one IF section. For example, some receivers employ two IF sections for recovering the transmitted information. These receivers are generally referred to as dual conversion receivers, whereas a single IF receiver would be referred to as a singular conversion receiver. Generally, any receiver with an intermediate frequency of zero Hertz is referred to as a direct conversion receiver.
Analog implementations of direct conversion receivers suffer from a variety of detriments including local oscillator (LO) radiation, which results from imperfect reverse isolation through the mixers, and may desense nearby receivers. Further, radio desense performance can be degraded by nonlinear effects in the mixers causing self-mixing of on-and off-channel signals which creates DC offsets and audio distortion. Also, in an application involving the reception of frequency modulated (FIR) signals, the direct conversion analog receiver has no means of limiting the zero-IF signal. This causes unpredictable performance in fading and other adverse conditions.
The aforementioned dual conversion receiver alleviates some of the direct conversion problems. The additional isolation obtained by a dual conversion receiver solves the LO radiation problem. However, the solution is achieved at the cost of an additional mixer and local oscillator, in addition to a narrow band (generally crystal) filter to achieve the required isolation. Further, having a traditional IF section prior to the DC-IF section essentially band-limits the incoming signal to one channel. Thus, the self-mixing products caused by the nonlinear effects of the mixers generally will not fall into the passband of the filter in a dual conversion receiver.
Although the dual conversion receiver solves many of the problems experienced by the direct conversion receiver (although at additional cost and size requirements), the dual-conversion receiver experiences other detriments. As previously mentioned, the direct conversion FM receiver cannot limit the zero-IF signal. Thus, the use of unconventional detection methods are required. The typical solution to this problem is to up-convert the zero-IF signal to a third IF frequency where it can be limited and detected using conventional circuits. Up-converting requires another local oscillator, additional mixers and a summing circuit.
Further, up-converting creates yet another problem. The quadrature paths in an analog receiver cannot be perfectly balanced for amplitude and phase characteristics because of the nonexact performance of the mixers and filters. Thus, a beat-note is created due to imperfect cancellation in the summer which degrades hum and noise performance and causes audio distortion. A proposed solution to this problem is to phase lock the LO to an incoming pilot signal. This necessitates additional circuitry at the transmitter to transmit the pilot signal and also requires additional circuitry at the receiver to develop the phase lock loop and pilot filters. Lastly, the phase lock loop lock-time and pull-in range become critical receiver parameters.
Even more recent attempts to resolve the above-mentioned problems using digital receiver implementations have failed to provide a practical, cost-effective arrangement. The hardware necessary to meet the selectivity section requirements should be minimized to lessen circuit board real estate and power consumption in the radio. Generally, however, a significant amount of parallel architecture is needed to accommodate the substantially high data rates required in such an implementation. Moreover, since such designs are often sought to be implemented within integrated circuits, hardware minimization is highly advantageous. Hence, when designing a selectivity section to resolve the above-mentioned problems, it is important to minimize the hardware and power consumption for a practical implementation, while still allowing operation at sufficiently high data rates.
Although the above discussion has concerned receivers, similar problems are experienced in transmitter IF sections, even though transmitter IF topologies are generally different than those employed in receivers. Generally, any analog implementation of an IF section will experience temperature and part-to-part variations that may degrade the IF section performance.
Therefore, a need exists for an IF section that is insensitive to part and temperature variations and solves the above mentioned problems experienced in analog implementations of IF sections.